In many cases, a pnm isn't her best "self" in the hectic, fast-paced environment of formal Rush. Formal is not the best way to get to know some women, like myself. COB is more comfortable for a lot of rushees, so the sororities get to see who a rushee really is. This happened to me, and it made perfect sense!
I'm used to clamming up in big crowds, and formal rush was no different. When I was extended a bid during COB, I felt confident that it was because the sisters got to know the real me.
LM